1. Field of the Invention
The present invention relates to an apparatus and method for controlling uplink power in a wireless communication system. More particularly, the present invention relates to an apparatus and method for controlling uplink power considering an interference between cells according to a cooperative transmission in a wireless communication system.
2. Description of the Related Art
A wireless communication system according to the related art employs a resource scheduling technique in order to efficiently use a limited radio resource. For example, the resource scheduling technique applied to the wireless communication system includes a resource allocation according to a transmission direction of data and a resource allocation considering reuse efficiency. Here, the resource allocation according to the transmission direction of the data includes a downlink resource allocation in which a resource is downlinked from a base station to a wireless terminal, and an uplink resource allocation in which a resource is uplinked from the wireless terminal to the base station. The resource allocation considering the reuse efficiency means that a resource in a service area of the wireless communication system is allocated considering an interference in an adjacent service area.
Representatively, the wireless communication system has employed techniques such as an Adaptive Modulation and Coding (AMC) scheme, a channel-sensitive scheduling scheme, and the like, in order to improve transmission efficiency.
The AMC scheme enables the wireless communication system to adjust an amount of transmission data according to a channel condition in order to transmit a large amount of information while maintaining a reception error probability at a desired level. For example, the AMC scheme can provide a scheduling for increasing or decreasing an amount of transmission data according to the channel condition.
The channel-sensitive scheduling scheme allows the wireless communication system to selectively provide service to a user, who has a superior channel condition, among several users. For example, the channel-sensitive scheduling scheme can relatively increases a system capacity since it enables the wireless communication system to selectively provide service to the user, who has the superior channel condition. The increase of the system capacity is generally referred to as a ‘Multi-user Diversity gain’.
The AMC scheme and the channel-sensitive scheduling scheme are to receive partial channel condition information being fed back from a receiver, and apply an appropriate modulation and coding technique at the most efficient time determined depending on the received partial channel condition information.
Where the AMC scheme is used along with a Multiple Input Multiple Output (MIMO) transmission scheme, it is possible to include a function of determining the number of spatial layers or ranking to correspond to a transmitted signal. In this case, the AMC scheme not only considers a coding rate and a modulation scheme simply, but also considers how many layers the data is transmitted in by using the MIMO, when an optimal data rate is determined.
As described above, the wireless communication system based on a cellular network considers a reuse efficiency when the resource is allocated. For example, when an identical frequency is used in an adjacent cell, a serious interference is generated in a boundary region between the cells. When the interference between the cells is enabled to be a minimum while a resource allocation is established to increase a use efficiency of a resource, it can be understood that the reuse efficiency is improved.
Therefore, various techniques have been proposed in order to improve the reuse efficiency of a radio resource in the wireless communication system.
In a case of a wireless communication system (hereinafter, referred to as a Code Division Multiple Access (CDMA) system) using a CDMA scheme, a base station transmits predetermined data to all terminals, based on a data circuit transmission. Accordingly, the base station determines only a data transmission rate of all terminals in a service area thereof. For example, the base station simultaneously adjusts the data transmission rate of all the terminals in a cell upwardly or downwardly when the data transmission rate of each terminal is determined. That is, the data transmission rate of all the terminals is adjusted depending on an inter-cell interference, an intra-cell interference, and a magnitude of a Rise over Thermal (RoT).
It makes the magnitude of the RoT in each cell to be maintained to an appropriate level so that a coverage area of each cell can be regularly maintained and the data transmission rate of each terminal can be maintained to an appropriate level. Here, the RoT is defined as a ratio of thermal noise to electric power received from all the terminals to the base.
On the contrary, a next generation wireless communication system including a Long Term Evolution (LTE), etc. supports a function of notifying other cells of a magnitude of interference in one cell at an uplink. Accordingly, each terminal analyzes an intensity, i.e., a Reference Signal Received Power (RSRP), of a downlink signal received from adjacent cells except for a serving cell, and calculates a downlink path loss of the serving cell and the adjacent cells. Each terminal transmits information on the calculated downlink path loss to the base station. The base station can perform a handoff of a corresponding terminal and an interference control for other cells.
On the other hand, the next generation communication system uses an Orthogonal Frequency-Division Multiplexing (OFDM) scheme. Hereinafter, a wireless communication system using the OFDM scheme is referred to as an ‘OFDM system’.
The OFDM system has only inter-cell interference, and has no intra-cell interference which is generated in the CDMA system. The reason is that the CDMA system distinguishes channels by using codes in an identical frequency band, while the OFDM system distinguishes channels by using subcarriers.
Accordingly, the next generation wireless communication system corresponding to the OFDM system should employ an electric power control technique different from that which the CDMA system employs to maintain an Interference over Thermal (IoT) at an appropriate level, in order to maintain the IoT below an appropriate level.
The electric power control technique which the wireless communication system generally has employed includes, for example, an Open-Loop scheme, a Closed-Loop scheme, and an Inter-Cell Interference Coordination (ICIC) scheme.
On the other hand, in the next generation wireless communication system corresponding to the OFDM system, not only an intra-cell interference but also an inter-cell interference must be considered in order to maintain the IoT of each cell below the appropriate level.
Therefore, the 3rd Generation Partnership Project (3GPP) LTE standard provision supports an ICIC scheme. The ICIC scheme adjusts a transmission power for each frequency resource considering a traffic and an interference of User Equipment (UEs) belonging to each cell for a long time.
Accordingly, the ICIC scheme prevents a signal from being transmitted through a certain frequency resource or decreases transmission power of a signal which uses the certain frequency resource, in an adjacent cell, thereby reducing an interference for the UE located on a boundary of a cell.
However, as described above, there is a limitation in that an inter-cell interference is reduced through a resource allocation and a transmission power control so as to improve a system capacity in a wireless communication system based on a cellular network.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.